Custom PSU question

[jbartee]

Hi all,

I've looked around the forums trying to find an answer to this and didn't see anything, but feel free to issue a prompt STFU if this is covered somewhere.

I'm currently building a MBSID in a keytar form factor using the original C64 case for the body. Since the old breadbox needs to house a keyboard in addition to most of the circuitry, I'm pretty short on space and thermal management is a big concern for me. Therefore I want to get all voltage regulators out of the case since these are the likely candidates for generating the most heat.

I've built an external power supply loosely based on the optimized PSU but using the guts from a couple modern AC adapters instead of the old c64 brick which I don't think would pump out enough amperage for what I want to do. My PSU supplies 5 and 12 volts regulated, in addition to a 9 volt line to run the midi keyboard.

The PSU is very basic and doesn't do anything to the power except output DC and regulate it with a 7805 and 7809. When I hook this up to Smash's core and sid modules, I will obviously leave his regulators unstuffed, and I suppose I should throw out the bridge rectifiers as well since there's no AC to deal with. But what about the various capacitors on the power rail (C4, C5, C6 on the core module for instance?) I'm guessing these are working as reservoir caps to reduce ripple? If so that's a good thing to have. Could I just supply my power directly upstream of C5 (core) or C9 (sid) and be done with it (jumpering the 7805/7805, of course)? Or is there something I'm missing that is going to lead me to blow up my boards? I'm moderately experienced with electronics, but more on the microcontroller level -the ins and outs of power management are somewhat elusive to me!

Thanks in advance,

Jordan

[Wilba]

It's a good idea to leave the capacitors in... as you're obviously supplying power to the modules from a long-ish cable, so the additional capacitors on the modules are good to have. I read somewhere that anything over two inches from the voltage regulator (even on tracks on the PCB) will require some caps at the end to keep things smooth.

I'm no expert on these things, but I designed the MB-6582 PCB and it seems to work... it regulates the 9V with a single 7809 and supplies it to eight SID modules/SIDs via one track, with a few 100nF in each module to keep things smooth. In addition, the MB-6582 takes an external regulated 5V and supplies the whole PCB, I'm using a 2200 uF electro plus the usual 100nF monolithic ceramics everywhere to keep the 5V smooth.

So you can take the same approach...

Have a look at this circuit:

http://www.ucapps.de/mbhp/mbhp_8xsid_c64_psu_optimized.pdf

It's slightly different to what I've done... as it supplies each SID module with 14V DC and then relies on the 9V or 12V regulator on each SID module to regulate it down to the SID's required voltage. However, use it as a guide to connecting all your modules to a common 5V supply, and/or a common 9V or 12V supply (depending on SID type), and then not stuffing the 9V or 12V regulators on the SID modules themselves.

[jbartee]

It's a good idea to leave the capacitors in... as you're obviously supplying power to the modules from a long-ish cable, so the additional capacitors on the modules are good to have. I read somewhere that anything over two inches from the voltage regulator (even on tracks on the PCB) will require some caps at the end to keep things smooth.

I'm no expert on these things, but I designed the MB-6582 PCB and it seems to work... it regulates the 9V with a single 7809 and supplies it to eight SID modules/SIDs via one track, with a few 100nF in each module to keep things smooth. In addition, the MB-6582 takes an external regulated 5V and supplies the whole PCB, I'm using a 2200 uF electro plus the usual 100nF monolithic ceramics everywhere to keep the 5V smooth.

So you can take the same approach...

Have a look at this circuit:

http://www.ucapps.de/mbhp/mbhp_8xsid_c64_psu_optimized.pdf

It's slightly different to what I've done... as it supplies each SID module with 14V DC and then relies on the 9V or 12V regulator on each SID module to regulate it down to the SID's required voltage. However, use it as a guide to connecting all your modules to a common 5V supply, and/or a common 9V or 12V supply (depending on SID type), and then not stuffing the 9V or 12V regulators on the SID modules themselves.

Thanks so much for the quick reply Wilba. This information is super helpful.

As a matter of general good practice, should I build in a 2200 uF electrolytic before the voltage regulator, in my external power supply? Or can the smoothing all take place after regulation on the PCB's? I suppose it couldn't hurt to have both.

Edited January 9, 2010 by jbartee

[lylehaze]

Thanks so much for the quick reply Wilba. This information is super helpful.

As a matter of general good practice, should I build in a 2200 uF electrolytic before the voltage regulator, in my external power supply? Or can the smoothing all take place after regulation on the PCB's? I suppose it couldn't hurt to have both.

Without seeing your schematics.. I can fairly safely say Yes, you need a cap before the regulator(s).

The "usual" sequence of power is Transformer, bridge, capacitor, then the regulator(s) then more caps.. without the cap after the bridge, the input voltage will fall to zero 120(or 100) times per second (depending on what country you're in). SO the cap before the regulator will "average" the pre-rectified voltage. It MUST be enough so that the input voltage never falls below the "dropout" voltage of the regulator.. about 7 to 7.5 volts for a 7805, or 14V for a 7812.

If you want to read more..

Have Fun!

LyleHaze

[jbartee]

Without seeing your schematics.. I can fairly safely say Yes, you need a cap before the regulator(s).

The "usual" sequence of power is Transformer, bridge, capacitor, then the regulator(s) then more caps.. without the cap after the bridge, the input voltage will fall to zero 120(or 100) times per second (depending on what country you're in). SO the cap before the regulator will "average" the pre-rectified voltage. It MUST be enough so that the input voltage never falls below the "dropout" voltage of the regulator.. about 7 to 7.5 volts for a 7805, or 14V for a 7812.

If you want to read more..

Have Fun!

LyleHaze

Thanks! That clarifies everything.

[Wilba]

Note that in the schematic I referenced, there is a 2200uF electro on the regulated 5V supply, i.e. after the 7805 (or alternative) regulator inside a C64 PSU brick. This (apparently) helps eliminate digital noise, so it's probably a good idea to put one after the 7805 on your PSU for the same effect. It's not needed after a 7809 or 7812 (i.e. for SID supply) because it's not supplying digital ICs so to speak, only the analog part of the SIDs and the audio buffer on the SID module.

[m00dawg]

You might look =psu]here as well for some ideas. The idea using using resistors to drop the voltage to the 7805 seems to be a good one and is what I use. You're not avoiding heat this way, but using resistors to dissipate some of it instead of trying to make the 7805 do it all.

My design fits in a plastic box but I plan on using a gutted PSU case for better heat dissipation since it has room for a fan and is also metal. So I can mount my resistors right to the chassis and I have an instant heat-sink.

I am a HUGE fan of custom PSUs because I hate the C64 brick of death. Maybe it is was remotely user serviceable without a circular saw it might be another story :) Plus, making PSUs is fun and is a good way to learn some useful information (though, as a disclaimer, you ARE dealing with main AC power so don't be stupid). The only downside is that it makes using the standard power switch that comes with the MB-6582 kit a bit more difficult. For the interim, I put a switch on my PSU directly (which is still a good idea, if a little inconvenient).

All that said, there's plenty of information about how (and how not) to make PSUs on the forum that you should find helpful. I tried to dig up a few but a quick search should give you plenty of stuff. The Wiki, too, has various designs.There's a lot of ways to go so it really depends on your needs and your plans. If you want to do the AOUT or SSN filtering stuff, for instance, you might want to factor in using a bi-polar supply.